Honeycomb structure and method for production of said structure

ABSTRACT

An alveolar structure and a method of manufacturing an alveolar structure. The alveolar structure includes at least one alveolar zone partially delimited by an associated leak tight surface. Each alveolar zone is formed of a plurality of metallic layers superimposed parallel to the associated leak tight surface, each metallic layer including a network of passages opening out on either side of the each metallic layer. The alveolar structure may find particular application to fuel cells and heat exchangers.

TECHNICAL FIELD

The technical field of the present invention concerns that of theproduction of energy requiring a high compactness of the componentsused. More specifically, the invention relates to alveolar structuresused in this specific technical field.

The alveolar structures find application, in particular, in the field offuel cells and, more specifically, in that of fuel cells comprising amembrane as electrolyte as well as bipolar plates, said plates beingcomposed of alveolar structures.

Furthermore, the invention is also applicable to the field of heatexchangers using alveolar structures.

Finally, the present invention further concerns methods formanufacturing said alveolar structures.

STATE OF THE PRIOR ART

In this field, fuel cells using alveolar structures are known.

Indeed, a fuel cell is an assembly generally comprising a plurality ofelementary cells stacked one against the other. In each of theelementary cells of the fuel cell, an electrochemical reaction iscreated between two reagents that are introduced in a continuous mannerinto the elementary cells. The fuel normally used is hydrogen ormethanol, depending on whether one is in the presence respectively of acell operating with hydrogen/oxygen type mixtures and in the presence ofa cell operating with methanol/oxygen type mixtures.

The fuel is brought into contact with the anode whereas the oxidant, inthis instance oxygen, is brought into contact with the cathode.

The cathode and the anode are separated by the intermediary of an ionexchange membrane type electrolyte.

At the level of the anode, an oxidation reaction of the fuel, generallyhydrogen, takes place, represented by the following reaction scheme:2H₂→4H⁺+4e ⁻

In the same way, at the level of the cathode, a reduction reaction ofthe oxidant, generally oxygen, takes place, according to the followingreaction scheme:O₂+4H⁺+4e ⁻→2H₂O

One then has an electrochemical reaction in which the energy created isconverted into electrical energy. Protons, H⁺, circulate from the anodein the direction of the cathode, crossing the electrolyte, to join upwith an exterior circuit in order to contribute to the production ofelectrical energy.

At the same time, at the level of the cathode, one has the production ofwater, which is continuously evacuated from theelectrode-membrane-electrode assembly.

In fuel cells of the prior art, several electrode-membrane-electrodeassemblies are stacked one against the other in order to obtain a higherpower to that provided by only one of said assemblies. The junction andthe electrical continuity between said assemblies is generally achievedby means of conductive plates, said plates also being called bipolarplates.

It is therefore by means of said bipolar plates, being of the alveolarstructure type, that one can join the cathode of one assembly with theanode of an adjacent assembly. Said bipolar plates further make itpossible to assure the highest possible electrical conductivities, insuch a way as to avoid ohmic drops that are detrimental to the output ofthe fuel cell.

The bipolar plates may also fulfil other functions to that of ensuringthe electrical junction.

Indeed, one may for example carry out, through the intermediary of saidbipolar plates, the continuous supply in reagent of the anode of a firstassembly and the cathode of a second adjacent assembly.

Moreover, the bipolar plates may also serve in the evacuation ofproducts at the level of the cathode, by integrating elements foreliminating the water in excess.

The bipolar plates may further incorporate a heat exchanger serving toavoid any overheating within the stack of electrode-membrane-electrodeassemblies.

It should finally be noted that another function of said bipolar platesmay reside in the mechanical strength of theelectrode-membrane-electrode assemblies, particularly when saidassemblies are stacked one against the other. This type of assemblyassures an overall volume of the cell of low thickness, which is fullycompatible with the planned applications, such as for example thatconcerning an electrical vehicle.

According to the devices and methods of the prior art, three distinctmethods exist for achieving the distribution of the reagents.

One notes firstly a method using channels machined in the ends of thebipolar plates. Said channels are provided to ensure the mosthomogeneous possible distribution of the reagents on a surface of theelectrode with which they are in contact.

Said channels are normally organised in such a way that the reagentsinjected into said channels wind along a large part of the surface ofthe electrode. The means implemented to obtain this type of result arehorizontal sections spaced by bends descending to 180°. It should benoted that said sections are also capable of recovering and evacuatingthe water produced at the level of the cathode.

However, it has been observed that this specific arrangement of meanswould not enable a sufficiently large exchange surface to be obtained tolead to an acceptable yield of electrochemical conversion with a view toan industrial application.

In order to make up for this disadvantage, a second method has beenproposed in the prior art.

This method involves using a high porosity metallic foam to add to themetallic parts in which are formed machinings, said metallic foam makingit possible to ensure a good distribution of the reagents and theevacuation of different products.

This type of method is, in particular, described in the document U.S.Pat. No. 5,482,792. Two foils of several millimetres thickness arerespectively positioned against the anode and against the cathode, andalso form the junction with the ends of the bipolar plate.

However, the fact of adding a metallic foam at the level of the bipolarplate contributes to creating a considerable resistance, which leads toa drop in the electrical conductivity within the assembly.

Even if the problem relating to the electrical conduction may bepartially resolved by compressing the metallic foam, it turns out that,whatever the case, problems of corrosion persist, notably due to thepresence of numerous defects such as strand ruptures within the metallicfoam.

According to third method known in the prior art, described in thedocument U.S. Pat. No. 6,146,780, the bipolar plate comprises a leaktight conductive plate, as well as two parts in metallic foam assuringthe contact with the electrodes. This specific arrangement makes itpossible to do without the presence of machined and, consequently,expensive metallic elements.

On the other hand, other disadvantages remain in the use of suchdevices.

Indeed, by using metallic foams as distribution zones for reagents andthe evacuation of different products, one cannot correctly control theperiodicity of the metallic foam type structure.

Furthermore, an additional disadvantage resides in the impossibility ofcontrolling, in an easy manner, the internal geometry of the alveolarstructure, this being reflected by the incapacity to vary the geometryof the distribution zone in the desired manner.

Finally, it should be pointed out that one again encounters some of theaforementioned disadvantages in the alveolar structures used in heatexchangers of the prior art.

DESCRIPTION OF THE INVENTION

The aim of the present invention is therefore to overcome all or part ofthe disadvantages of alveolar structures of the prior art.

The further aim of the invention is to propose an alveolar structure ofsimple design, and for which it is possible to perfectly control theinternal geometry of its different alveolar zones.

A yet further aim of the present invention is a method of manufacturingan alveolar structure such as that fulfilling the aim of the inventionmentioned above.

To do this, a first aim of the invention is an alveolar structurecomprising at least one alveolar zone partially delimited by anassociated leak tight surface. According to the invention, each alveolarzone is formed of a plurality of metallic layers superimposed parallelto the associated leak tight surface, each metallic layer comprising anetwork of passages opening out on either side of said metallic layer.

Advantageously, the invention proposes an alveolar structure of simpledesign in which the internal geometry of the alveolar zones is easilyadaptable, depending on the encountered needs.

In this type of alveolar structure, one does not encounter anydifficulty linked to assemblies of elements provoking mechanical,thermal or electrical discontinuities within the bipolar plate.

Preferentially, the alveolar structure is indiscriminately integrated ina fuel cell as a bipolar plate, in a fuel cell as a bipolar plate withintegrated exchanger or even integrated in a heat exchanger.

A further aim of the present invention is a method for manufacturingsaid type of alveolar structure. According to this method, each metalliclayer is formed by carrying out the following operations:

-   -   depositing a layer of metallic powder;    -   partially solidifying by laser the layer of deposited metallic        powder, leading to the formation of solidified parts and        non-solidified parts, said solidified parts defining the        perimeter of the passages of the metallic layer.

Preferentially, for each alveolar zone, the metallic layers are formedsuccessively, the associated leak tight surface constituting a supportfor the first metallic layer to be formed, the solidified andnon-solidified parts of any metallic layer formed constituting a supportfor the following metallic layer to be formed.

Preferentially, for each alveolar zone, when all of the metallic layershave been formed, the networks of passages of the metallic layers areobtained by eliminating the non-solidified parts of the metallic layers.

Furthermore, one may provide that the operation of partialsolidification by laser of a layer of deposited metallic powder is alsocapable of making the solidified parts obtained integral with thesolidified parts of the metallic layer on which they are lying.

The metallic layers are preferably composed of a material chosen fromamong stainless steels, aluminium and its alloys, nickel and its alloyssuch as Ni—Cr, and a mixture of at least two of the aforementionedelements.

Furthermore, the metallic layers comprise at least one binder such asbronze. This advantageously makes it possible to obtain alloys for whicha sintering operation can be carried out at low temperature.

Other advantages and characteristics of the invention will become clearin the detailed, non-limitative description that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

The description will be made with respect to the appended drawings, inwhich:

FIG. 1 represents a perspective view of an alveolar structure accordingto a preferred embodiment of the invention,

FIG. 2 represents a frontal view of the alveolar structure of FIG. 1, incontact with two elements to be linked together,

FIG. 3 represents a frontal view of an alveolar structure according toanother preferred embodiment of the invention,

FIG. 4 schematically represents a perspective view of an alveolar zoneduring manufacture, after the step of depositing a layer of metallicpowder, and

FIG. 5 schematically represents a perspective view of an alveolar zoneduring manufacture, after the step of partial solidification of a layerof deposited metallic powder.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

With reference to FIGS. 1 and 2, an alveolar structure 1 according to apreferred embodiment of the invention is shown, said alveolar structurebeing, in particular, capable of functioning with a fuel cell or with aheat exchanger (not shown).

The alveolar structure 1 according to the invention comprises at leastone alveolar zone 2 a, 2 b intended to be crossed by at least one fluid.According to the preferred embodiment of the invention represented inFIGS. 1 and 2, the alveolar structure 1 comprises two alveolar zones 2a, 2 b, each being intended to cooperate with a respective element 16 a,16 b comprising a surface to contact 14 a, 14 b. The elements 16 a, 16 bvisible in FIG. 2 may belong to a heat exchanger or a fuel cell.

It should be noted that it is also possible to propose an alveolarstructure 1 only having one alveolar zone 2 a, 2 b.

The alveolar zones 2 a, 2 b are partially delimited by associated leaktight surfaces 4 a, 4 b.

The first 4 a as well as the second 4 b belong to a conductive baseplate 6, said base plate 6 also being leak tight to fluids flowingwithin the alveolar zones 2 a, 2 b.

Each alveolar zone 2 a, 2 b is formed of metallic layers 8 superimposedparallel to the associated leak tight surface 4 a, 4 b. Each metalliclayer 8 comprises a network of passages 10 opening out on either side ofthe metallic layer 8.

In each alveolar zone 2 a, 2 b, the metallic layers 8, substantiallyplanar, are therefore stacked one against the other, on a large part ofthe associated leak tight surface 4 a, 4 b. Each metallic layer 8comprises a network of passages comprising a plurality of passages 10,of identical or different shape, crossing each metallic layer 8 along anaxis substantially perpendicular to the associated leak tight surface 4a, 4 b. This specific arrangement of the metallic layers 8 thereforeleads to obtaining volumic, conductive and porous alveolar zones 2 a, 2b.

Each of the metallic layers 8 may have an identical or different networkof passages 10 of networks of passages 10 formed on the two directlyadjacent metallic layers 8.

Preferably, and with reference to FIG. 2, the alveolar structure 1 has athickness “E” of around 6 mm, said thickness corresponding to thethickness “e” of the conductive base plate 6 added to the sum of theheights “ha” and “hb” of the two alveolar zones 2 a, 2 b.

Furthermore, still with reference to FIG. 2, the metallic layers 8 maybe of different thickness such as “e′” or “e″”, these values beingpreferably less than 0.5 mm and, more specifically, between around 0.1mm and 0.2 mm.

Thus, by varying firstly the thicknesses “e′” and “e″” of the metalliclayers 8, and secondly the distribution and the geometry of the networksof passages 10, one may obtain alveolar zones 2 a, 2 b of average openporosity between a value strictly superior to 0% and 90%.

In operation, and with reference to FIGS. 1 and 2, the alveolarstructure 1 comprises two alveolar zones 2 a, 2 b, each being intendedto cooperate with a distinct surface 14 a, 14 b respectively belongingto the elements 16 a, 16 b. The elements 16 a, 16 b may belong to a heatexchanger or a fuel cell. Each alveolar zone 2 a, 2 b is respectivelysupplied with a fluid F₁ in the alveolar zone 2 a, and by a fluid F₂ inthe alveolar zone 2 b. It should be pointed out that said fluids F₁ andF₂ may themselves be mixtures of several fluids and that the supply offluids is preferably carried out in a continuous manner.

The arrows A and B respectively symbolise the supplies in fluids F₁ andF₂, respectively carried out in the alveolar zones 2 a and 2 b.

During their introduction into the alveolar structure 1, the fluids F₁and F₂ flow in the totality of the volume of the alveolar zones 2 a, 2 band diffuse up to the surfaces 14 a, 14 b with which they have to comeinto contact. The arrows C₁ and C₂ indicate a principal direction ofdiffusion of the fluids F₁ and F₂ in each of the alveolar zones 2 a, 2b.

To reach the surfaces 14 a, 14 b to be contacted, the fluids F₁ and F₂pass through the passages 10 formed in the metallic layers 8. With thistype of arrangement of means, the distribution of the fluids F_(1 and F)₂ on the surfaces to be contacted is assured to be as homogeneous aspossible.

The evacuation of the fluids F_(1 and F) ₂ is respectively symbolised bythe arrows D_(1 and D) ₂.

According to a first application of the invention, the alveolarstructure 1 is intended to be used in a fuel cell. In this specificcase, the two alveolar zones 2 a, 2 b are reagent distribution zones,and the surfaces 14 a, 14 b that have to contact these distributionzones are electrode surfaces 16 a, 16 b each belonging to anelectrode-membrane-electrode assembly (not shown) of a cell of a fuelcell. The alveolar structure 1 is then a bipolar plate for a fuel cell.

In the same way as in the bipolar plates of the prior art, thedistribution zones made up of the alveolar zones 2 a, 2 b may also servefor the evacuation of different products, such as water, formed duringthe electrochemical reactions at the electrodes.

It should also be noted that the alveolar structure 1, depending on theneeds encountered, may only comprise a single reagent distribution zone.This specific embodiment arises in cases where a single electrode of afuel cell is to be supplied with reagent.

Furthermore, with reference to FIG. 3 and according to a secondapplication of the present invention, by combining two alveolarstructures 100 and 200, one can obtain a bipolar plate comprising anintegrated heat exchanger.

Indeed, a first alveolar structure 100 comprising two alveolar zones 102a, 102 b is juxtaposed to a second alveolar structure 200 comprising asingle alveolar zone 202 a. One can bring said alveolar structures 100,200 into contact with each other, for example by simple pressing, whichforms a structure comprising three distinct alveolar zones 102 a, 102 b,202 a, the alveolar zone 102 a located in the middle of the two othersbeing a heat exchange zone and the two other zones 102 b, 202 a, locatedat the ends of the assembly, correspond to zones for distributingreagents to the electrodes.

According to a third application of the invention, the alveolarstructure 1 may also be used in a heat exchange type device, as a heatexchange zone. Its operation is then similar to that of bipolar plates,and the fluids injected into the alveolar zones 2 a, 2 b are a coolingliquid, such as water.

In this type of application, the alveolar zones 2 a, 2 b are coolingliquid distribution zones, said cooling liquid being intended to spreadout over the totality of the surfaces 14 a, 14 b to be cooled, then tobe evacuated from the alveolar structure 1 along a direction representedby the arrows D₁ and D₂ of FIG. 1.

The invention further relates to a method of manufacturing an alveolarstructure 1, as describe here above.

Said method of manufacturing consists, from the conductive base plate 6,in forming at least one alveolar zone 2 a, 2 b.

With reference to FIGS. 4 and 5, to result in one of the alveolar zones2 a, 2 b, one repeats two successive operations several times, enablingone to result in the formation of a metallic layer 8. With reference toFIG. 4, the first operation consists in depositing a layer of metallicpowder 18 on the last metallic layer 8 that has just been deposited. Itshould be noted that for the formation of the first metallic layer 8,this first operation consists in depositing a layer of metallic powder18 on the associated leak tight surface 4 a, 4 b.

Subsequently, the operation consists in partially solidifying, by laser,the layer of deposited metallic powder 18, in such a way as to obtainsolidified parts 20 and non-solidified parts 22. It is pointed out thatthe non-solidified parts 22 are formed of particles of powder of thelayer of metallic powder 18.

The location and quantities of solidified parts 20 and non-solidifiedparts 22 are determined as a function of the desired network of passages10 on the metallic layer 8 being formed. Indeed, the solidified parts 20define the perimeter of the passages 10, whereas the location of thenon-solidified parts 22 correspond to the desired location for thepassages 10 of said metallic layer 8.

For each alveolar zone 2 a, 2 b, one thus repeats this succession of twosteps, and this is done as many times as there are metallic layers 8constituting the alveolar zone 2 a, 2 b concerned.

It should be noted that after the formation of any metallic layer 8,said layer has solidified parts 20 and non-solidified parts 22, thewhole of these parts 20, 22 constituting a support for carrying out thedeposition of the following metallic layer 8.

In order to deposit the layers of metallic powder 18, one may resort toany type of method known to the prior art. Preferentially, onemechanically deposits the layers of metallic powder 18.

In order to carry out the step of partial solidification of the layer ofdeposited metallic powder 18, one uses methods known to the prior artand using laser type means.

By way of example, on may cite selective laser sintering, direct powderdeposition, rapid fabrication and production, laser sintering or evenproduction of microsystems.

In a general manner, the methods mentioned above use laser type means toprovide locally sufficient power to sinter or melt part of the layer ofmetallic powder 18, at a precise and predetermined position. Thisoperation may be carried out several times in order to obtain aplurality of solidified parts 20.

It is then imperative to carry out a precise positioning of the lasertype means in relation to the layer of metallic powder 18 intended toundergo the partial solidification operation, in such a way that saidlayer of metallic powder 18 is located in a focal zone of the laser typemeans. Thus, in certain cases encountered, one is in a position tosolidify part of the layer of powder 18 without solidifying thenon-solidified part(s) 22 of the metallic layer 8 on which it is lying.

It should be noted that it is possible to carry out this method with theaid of CAD type means, said means making it possible to adjust, whilethe method is in progress, the positions relative to the laser typemeans and the different layers of metallic powder 18.

It should also be pointed out that the operation of partialsolidification by laser of a layer of metallic powder 18 also makes itpossible to make the solidified parts 20 obtained integral with thesolidified part(s) of the metallic layer 8 on which they are lying. Thischaracteristic is also valid for the first metallic layer 8 formed, thesolidified parts 20 obtained being made integral with the associatedleak tight surface 4 a, 4 b of the plate 6.

When the assembly of metallic layers 8 intended to form an alveolar zone2 a, 2 b has been formed, one then has a block comprising exclusivelysolidified parts 20 and non-solidified parts 22. Thus, in order toobtain the networks of passages 10 located at the level of thenon-solidified parts 22, it is necessary to eliminate the powderconstituting said parts 22. This elimination may quite simply be carriedout by evacuation of the grains of powder, said grains being easilyextractable from the block, creating, as they are extracted, thenetworks of passages 10 of the metallic layers 8.

After elimination of all of the non-solidified parts 22, the alveolarzone 2 a, 2 b is obtained, formed of a plurality of solidified parts 20that are made integral with each other.

Obviously, various modifications may be made by those skilled in the artto the alveolar structure 1 and to the method of manufacturing said typeof structure that has just been described, uniquely by way of exampleand in nowise limitative.

1-11. (canceled)
 12. An alveolar structure comprising: at least one alveolar zone partially delimited by an associated leak tight surface, wherein each alveolar zone is formed of a plurality of metallic layers superimposed parallel to the associated leak tight surface, each metallic layer comprising a network of passages opening out on either side of said each metallic layer.
 13. An alveolar structure according to claim 12, integrated in a fuel cell as a bipolar plate.
 14. An alveolar structure according to claim 12, integrated in a fuel cell as a bipolar plate with an integrated exchanger.
 15. An alveolar structure according to claim 12, integrated in a heat exchanger.
 16. A method of manufacturing an alveolar structure according to claims 12, wherein each metallic layer is formed by: depositing a layer of metallic powder; partially solidifying by laser the layer of deposited metallic powder, leading to formation of solidified parts and non-solidified parts, the solidified parts defining a perimeter of the network of passages of the metallic layer.
 17. A method according to claim 16, wherein for each alveolar zone, the metallic layers are formed successively, the associated leak tight surface constituting a support for a first of the metallic layers to be formed, the solidified and non-solidified parts of any metallic layer formed constituting a support for following of the metallic layers to be formed.
 18. A method according to claim 16, wherein for each alveolar zone, when all of the metallic layers have been formed, the network of passages of the metallic layers are obtained by eliminating the non-solidified parts of the metallic layers.
 19. Method according to claim 16, wherein the partially solidifying by laser of a layer of deposited metallic powder is also capable of making the solidified parts obtained integral with the solidified parts of the metallic layer on which they are lying.
 20. A method according to claim 16, wherein the partially solidifying by laser of the first layer of deposited metallic powder is also capable of making the solidified parts obtained integral with the associated leak tight surface on which they are lying.
 21. A method according to claim 16, wherein the metallic layers are composed of a material chosen from among stainless steels, aluminium and its alloys, including Ni—Cr, and a mixture of at least two of the aforementioned elements.
 22. A method according to claim 21, wherein the metallic layers comprise at least one binder.
 23. A method according to claim 21, wherein the at least one binder comprises bronze. 